1. Field of the Invention
This invention is concerned with the arrangement of electrodes within the print head of a non-impact printer. More particularly, this invention is directed to improve print head apparatus having improved operating and enhanced wear characteristics.
2. Description of the Prior Art
The concept of electrically generated printing had sparked interest since the 1840's. Most attempts at utilizing an electrically initiated printing reaction required relatively high voltages, in the order of 100 to 250 volts, saturated or partially wetted paper and/or consumable electrodes. It was also necessary to employ a special recording medium which would be suitable for the particular printing system being used. Almost all of the prior art systems relied on either relatively high or long amplitude voltage pulses to achieve "dry" or "wet" printing. As might be expected, there were also hybrid systems and recording medium therefor that attempted to reconcile and/or compensate for the disadvantages of both the dry and wet approaches. However, as is the case with most compromise situations, these efforts were either too expensive to commercially implement or unsatisfactory in performance.
Various efforts were made to improve different aspects of the prior art electrically induced printing systems. Such efforts led to the use of electrolytic based printers wherein a relatively low voltage, on the order of no more than 25 volts, is employed to effect printing. One printing system that functions at low energy printing levels, of a magnitude that is compatible with today's densely populated integrated circuit chips, is described in the commonly assigned U.S. Patent Application Ser. No. 237,560 filed on Feb. 24, 1981 in the name of Bernier et al. In this printing arrangement, a leuco dye resident in or applied to the surface of the recording medium used therein is rendered visible by the application of a low energy pulse, providing that the surface layer thickness of the recording medium, the contact surface area of the print electrodes and the spacing therebetween are all set to predetermined values.
It has been found that the specific configuration of the multiple electrode print head plays an important role in overall performance in printers of this type. Also of importance is the particular material that the electrodes are made of and the manufacturing methods used to produce the print head. Print heads used in such electrolytically implemented printers must be able to withstand considerable electrolytic and electrochemical wear forces, the low print voltages notwithstanding. This aspect of print head life is in addition to the normal mechanical, principally frictional wear that a print head must tolerate. In addition, it has been determined that proper electrode configuration and isolation aids in preventing secondary or latent printing by trailing electrodes.
With respect to this type of printing, it has been determined that there are certain electrode materials that have good to excellent overall wear characteristics, provided that they are utilized properly. Platinum is one example of such an electrode material. It has excellent head wear characteristics under electrochemical and electrolytic stress, but must be correctly employed in order to benefit from its use. Specifically, the superior wear characteristics of platinum are attributable, at least in part and to the extent germane to this invention, to a passivation or barrier layer which is formed on its exposed surfaces. It is this layer that serves in large measure to minimize electrode wear when platinum electrodes are used in an electrolytic printer.
In use, however, it becomes evident that platinum electrodes were wearing unusually fast in print heads where they were not electrically isolated, at a rate that was far in excess of that expected. The actual cause of this problem was found to be electrode reversal in the print head, a situation where an electrode was allowed to function as both an anode or cathode, depending on whether or not it was printing during particular portion of the print cycle. The change in current flow that resulted from this electrode function reversal had the unfortunate result of reducing or even destroying the passivation layer on the platinum electrodes. The solution, and the basis of the present invention, was to permanently force non-reversible roles on the print head electrodes.
One possible response to this problem is disclosed in U.S. Pat. No. 3,898,674 to Koch, although not specifically articulated as such in the reference. The Koch print head incorporates multiple electrodes, each being surrounded by an annular insulating ring which serves to separate the individual electrodes from a common ground. In performing this function, the insulating rings prevent any current flow into a particular electrode and thereby preclude said electrodes from reversing their roles as printing styli or cathodes. Unfortunately, it has been found that it is particularly difficult to build a print head in accordance with the teachings of Koch. For example, the insulating ring that surrounds each electrode must provide a perfect electrical seal and prevent all reverse current flow or the electrode will exhibit excessive wear and premature failure before too long. Print heads having such perfectly formed and uniform insulating rings have been difficult and expensive to manufacture. Thus, while the theoretical solution to this electrode wear problem is know, a practical solution thereto remains to be implemented.